Observational Studies Of High Redshift Protoclusters

Galaxy clusters are the largest gravitationally bound system and are the link be-tween galaxies and large-scale structure of the Universe.How galaxy clusters and member galaxies evolve from high-redshift to the present day is the key to testing the theoretical models of cosmology and structure formation,and understanding the influ-ence of the environment on the galaxy formation and evolution.Galaxy protoclusters at high redshifts,as the progenitors of the present-day galaxy clusters,appear as overdense structures that have not yet virialized.Their member galaxies can contribute more than 20%to the cosmic star formation rate density of the high-redshift Universe.Redshift z=2-3 is the critical epoch for the formation of massive galaxy clusters(>1015 M?),also the peak of cosmic star formation and Active Galactic Nucleus(AGN)activity.This dissertation focuses on search and confirmation of the most massive protoclusters during this period,and studies the characteristics of their member galaxies.Multiple background quasars in the same region can be used to measure the Lya ab-sorption produced by the intergalactic medium(IGM)in an massive overdense structure.The effective optical depth of the Lya absorption traces the total mass of the strucure.BOSS 1244 and BOSS 1542 descibed in this dissertation are selected from the SDSS quasar database based on this technology.Using narrowband technology of the wide-field infrared camera(WIRCam)of the Canada-France-Hawaii Telescope(CFHT),244 and 223 Ha emitter candidates(HAEs)are detected in the BOSS 1244(?417 arcmin2)and BOSS1542(?399 arcmin2)fields,respectively.It confirms that BOSS1244 and BOSS 1542 are extremely overdense.Considering that the HAE candidates are contam-inated by the foreground and background galaxies,we need to identify these emission line galaxies by near-infrared(NIR)spectroscopy.Using MMT/MMIRS and LBT/LUCI NIR multi-object spectrograph,we confirm 46 and 36 HAEs in the BOSS 1244(?55 arcmin2)and BOSS 1542(?61 arcmin2)fields,respectively.BOSS 1244 displays a South-West(SW)component at z=2.230 ± 0.002 and another North-East(NE)component at z=2.246±0.001 with the line-of-sight velocity dispersions of 405±202 km s-1 and 377±99 km s-1,respectively.Interestingly,we find that the SW region of BOSS1244 contains two substructures in redshift space,likely merging to form a larger system.In contrast,BOSS 1542 exhibits an extended filamentary structure with a low velocity dispersion of 247±32 km s-1 at z=2.241 ±0.001,providing a direct confirmation of a large-scale cosmic web in the early Universe.The galaxy overdensities ?g on the scale of 15 cMpc are 22.9±4₉,10.9±2.5,and 20.5±3.9 for the BOSS1244 SW,BOSS1244NE,and BOSS1542 filament,respectively.They are the most overdense galaxy protoclusters(?g>20)discovered to date at z>2.These systems are expected to become virialized at z?0 with a total mass of Msw=(1.59±0.20)× 1015 M?,MNE=(0.83±0.11)× 1015 M? and Mfilament=(1.42 ±0.18)× 1015 M?,respectively.Our results suggest that the dense substructures of BOSS 1244 and BOSS 1542 will eventually evolve into the Coma-type galaxy clusters or even larger.Together with BOSS1441,these extremely massive overdensities at z=2-3 exhibit different morphologies,indicating that they are in different assembly stages in the formation of galaxy clusters.Furthermore,we find two quasar pairs in BOSS1441,one quasar pair in BOSS1244 and BOSS1542,these quasar pairs together with CoSLAs may trace the most massive large-scale structures.We analyze the properties of HAEs in BOSS 1244 and BOSS 1542 based on the existing deep optical and NIR imaging data.We find that these galaxies in high density environment at z=2.24±0.02 tend to be redder and have higher stellar mass than those in low density environment.We find numbers of red HAEs with z-Ks>2.5,and their number density is 5-6 times that in the general field at the same epoch.We find that these red HAEs prferentially concentrate in higher density regions with the stellar masses>1011M?,suggesting that these galaxies have already formed a large part of their stellar mass before z?2.Interestingly,in the existing HAE spectra,we find two red massive galaxy pairs in the SW and NE regions of BOSS1244.In the SW region,we find two HAEs with very large stellar mass with>1012M?,which are detected by JCMT-850 ?m,implying that they may be the progenitor of the local brightest cluster galaxies.We examine morphologies of HAEs using HST F160W imaging data,finding that some red HAEs exhibit irregular morphology.We suspect that these HAEs may be dusty starburst galaxies.